.square {
    width: 100px;
    height: 100px;
    font-size: 100px;
    line-height: 100px;
    text-align: center;
}

#tic-tac-toe table, th, td {
    border: 1px solid black;
}

#game {
    margin: 0 auto;
    width: 300px;
}